1. Field of the Invention
The present invention relates to an improved shielded magnetometer. It is used in the precise measurement of slowly variable magnetic fields and in particular the earth's magnetic field, whose value is approximately 2.3.multidot.10.sup.-5 to 7.multidot.10.sup.-5 Tesla, as a function of the latitude.
2. Discussion of the Related Art
The magnetometer according to the invention is either of the nuclear magnetic resonance oscillator type (NMR), or of the electronic paramagnetic resonance oscillator type (EPR). Such magnetometers are known and are e.g. described in French Patent Application FR-A-2 583 887 and FR-A-2 603 384. Therefore there is no need to describe them in detail here.
However, it is pointed out that the operation of a magnetometer is based on the principle of the magnetic resonance of the protons or electrons which, in the presence of a magnetic field B, precess around the direction of said field at a frequency F, called the Larmor frequency and which is proportional to the modulus of B: ##EQU1## in which .gamma. designates the gyromagnetic ratio of the proton or electron.
Bearing in mind the values assumed by the earth's field, the resonant frequencies observed are between 1000 and 3000 Hertz for NMR magnetometers and between 0.7.multidot.10.sup.6 Hertz for EPR magnetometers.
The performance characteristics of these magnetometers can be reduced through the presence of interfering signals in the measuring range. These can be harmonics of industrial frequencies (50 or 400 Hz) for NMR magnetometers or radiowaves for EPR magnetometers.
Moreover, the presence of a significant field gradient (exceeding 100 nonoTesla/m) leads to a deterioration in the performance characteristics of a NMR magnetometer. The EPR magnetometer is much less sensitive to the gradient and the gradient must exceed 50 micro Tesla/m for there to be a deterioration in the signal.
Thus, if it is wished to optimize the operation of these magnetometers, they should be protected as far as possible against interfering signals, whilst still permitting the measurement of the slowly variable or continuous (d.c.) fields (0 to 1 Hz) like the earth's magnetic field.
This technical problem of providing a protection against alternating or a.c. electromagnetic fields, whilst still permitting the transmission of slowly variable or d.c. fields has a known solution in the shielding of the probe by a passive electromagnetic screen.
The attached FIG. 1 shows in exemplified manner a NMR magnetometer probe shielded according to the arrangement described in FR-A-2 583 887. As shown, the probe comprises two bottles 20, 22 with a radial solution. These two bottles are externally coated with a silvered metal coating respectively 30 and 32, subdivided into non-contiguous sectors (to avoid circumferential eddy currents). The probe also incorporates a central conductor formed from two half-sections 33a and 33b. The left-hand end 34 of the section 33a is connected to the central core of a coaxial supply cable 36, whilst the right-hand end 38 of section 33b is connected to the conductive layer 32 of the bottle 22 by conductive strips 39. A tuning capacitor 50 is provided between the sections 33a and 33b. The sampling and exciting windings of said probe comprise two hemispherical windings 40, 42 outside the resonator and two flat coils 44, 46 inside the resonator.
The shield of said probe is cylindrical and is constituted by metal, e.g. silver strips 48 deposited on an insulating support.
Although satisfactory in certain respects, said means suffers from a disadvantage with respect to the shield. Thus, the measurement of variations of the earth's field aims at detecting magnetic anomalies, which e.g. indicate an immersed ferromagnetic structure (ship or the like). The measurements are performed remotely, the probe being located on an aircraft or helicopter. However, such carriers are obviously exposed to rotary movements (roll, pitch and yaw). These movements when applied to the magnetometer and it shield lead to the appearance of an interference magnetic field induced by the eddy currents flowing in the shield. This interference field disturbs the measurement of the earth's field and is prejudicial to the detection of anomalies.
In general terms, the aim is that when the shield probe is rotated, its performance characteristic are not deteriorated by the presence of an induced interference magnetic field and this is the function of the shield. The Expert having no manufacture the same will generally give it a cylindrical shape, as in the example of FIG. 1, or possibly a parallelepipedic shape, because these shapes are easy to obtain.